FIG. 1 discloses the antenna contact 20 in the back casing of the mobile phone 10. The antenna contact is used to connect the mobile phone 10 with the external antenna of a vehicle (e.g. a car) in order to be able to receive and transmit radio signals with high signal quality. When the mobile phone 10 via the antenna contact 20 is plugged onto the external antenna connector arranged in the car panel, the antenna of the mobile phone is disconnected and the external car antenna (normally situated on the car roof) is connected to the mobile phone.
However, when the antenna contact is not in use, which is the normal case (hand held mode), the antenna contact is an open road for ESD (Electro Static Discharge) pulses, which propagate from the outside of the phone through the antenna contact to the electrical components (front end components) inside the phone, wherein these ESD-pulses adversely affect these components. Components that are very sensitive to ESD-pulses are e.g. the antenna switch, SAW-filter, and PIN-diodes. It should be realised that even though the antenna contact does not have any galvanic contact with the phone antenna (circuit board) the ESD-pulse will propagate this way. Laboratory tests have shown that a very common reason for the mobile phone to break is that ESD-pulses from the antenna contact have destroyed some RF (Radio Frequency) components.
Electro static fields generating the above ESD-pulses could for example emanate from the user of the phone. When the fingers of the user hand are rubbing the phone casing the fingers are electrically charged implying a discrepancy in voltage potential between the fingers and the phone casing, which could result in an electrostatic discharge.
Another dangerous situation, which could create ESD-pulses, is when the mobile phone is to be plugged onto the external antenna connector in the car panel. When the phone approaches the external antenna connector in the car panel, there may be an electrostatic discharge from the external antenna connector to the mobile phone due to a potential discrepancy between the mobile phone and the external antenna connector. This discharge could damage the components in the mobile phone.
The above discussed ESD problems are usually solved with some kind of electrical components, which take care of high voltages either by absorbing the pulses arising from the voltages or leading them to ground. Up to now these problems have been solved by different arrangement of PIN-diodes, zener-diodes, shottky-diodes, and varistors.
However, these arrangements have several drawbacks. For example, they tend to distort signals passing through them. The arrangement of diodes causes losses of inserted signals.
Another drawback with the arrangement is that a certain voltage level must be defined. Above this level the arrangement should discharge the voltage and below this level the voltage should be left unaffected. The problem is to define this voltage level. Since there is a voltage level the ESD protection will not work for voltage pulses below said level.
Yet another problem is that for high power ESD pulses, the ESD protection components will break.
Thus, an object of the present invention is to solve the above discussed problems and overcome said drawbacks.